I aquired a 20gal Stainless Steel fermentor, but i am not really interested in making batches bigger than 5 gallons. This fermentor came from a hospital and was used to store steral scrub water for operating doctors. It has a bowl bottom with a spigot mounted on it (like a conical, but bowlical) it has a sight glass and various other threaded ports (standard thread) mounted in the side. the lid is an air tight lid that locks shut. I am wondering if i can use this for 5 gallon batches, or if i cant i would be willing to trade for a smaller vessel that would be more useful to me.

Everyone I've talked to has said that as long as you follow good pitching procedures and have short lag times, the headspace is a non-issue. Honestly though, Even if you had 24-48 hr lag time I dont think it would really be that big of a deal. Think of how hard it is to properly aerate your wort by shaking the hell out of it. I can't see any oxidation from sitting calmly underneath for a few hours. It's going to fill with CO2 soon enough.

If you were concerned, you could purge the tank with CO2 before filling with wort. Eliminate any chance of oxidation from the get go.

just my .02

Dishonorable men like running water,
Follow the path of least resistance...

I don't see why you can't use the large vessel to ferment in as long as you take precautionary measures like keeping it sanitized and purging the racked wort with CO2. It would probably mean more work for you than a 6 gallon bucket or carboy, and a higher cost of using all that CO2, but it can be done. Some commercial breweries still ferment in an open fermenter where no top is on their vessels, and they put out some mighty good beers with that method. You just need a big yeast starter and an active one to insure that your fermentation starts with minimal lag time.

Personally though, I would think that I would try to avoid any excess work involved and still use a bucket or carboy. A 5 gallon batch is not a lot of volume or work to make. Save the large vessel for the time when you are brewing larger batches. I wish I had a vessel that size, as I do brew on occasion 20 gallon batches....but not too often. Actually, i'm thinking of downsizing my brewery, as I really enjoy brewing 5 gallon batches. It allows me to brew more often and it is easy to brew 2 different batches on each brew day.